1. Field of the Invention
The present invention relates to a dampening system used in printing presses. In particular, the present invention relates to systems for applying a dampening agent to a plate cylinder.
2. Description of the Prior Art
In the field of offset printing presses, it is known to provide a system which is used to dampen the rolls or cylinders used in transferring an inked image to a web or signature. In general, a dampening fluid, such as water or water mixed with other fluids, is transferred to one or more rolls or cylinders of the press, in order to prevent the ink from drying and therefore to ensure effective printing of the image on the web or signature.
Typically, at least one of two techniques are used to transfer dampening fluid to the rolls or cylinders of a printing press. In the first technique, the dampening fluid is sprayed, in the form of droplets, directly onto one of the rolls or cylinders in the printing press. In the second technique, a dampening train is used to transfer dampening fluid to one of the rolls or cylinders in the printing press. A dampening train is one or more rolls closely adjacent to one another in series, which transfer dampening fluid to subsequent, adjacent rolls, and then to the roll or cylinder in the printing press. The dampening fluid is either sprayedxe2x80x94in the form of dropletsxe2x80x94onto one of the rolls in the dampening train, or transferred by the rotation of a roll partially immersed in a dampening fluid source. Dampening trains, because of the number of rolls needed to smooth and transfer the dampening fluid, can be expensive and difficult to maintain.
Several systems have been developed in which the dampening fluid is applied directly to a roll or cylinder using spray nozzles. U.S. Pat. Nos. 4,198,907; 4,649,818; 4,815,375; 4,932,319; 5,025,722 and 5,595,116 each describe systems in which a dampening fluid is sprayed onto a roll. In these spray dampening systems, a series of spray nozzles is used to spray dampening fluid, with each nozzle spraying fluid as a mist or droplets in a conical or fan-like pattern. The nozzles are oriented relative to one another so that the spray pattern of each individual nozzle is closely adjacent to each other nozzle, such that the series of nozzles result in full coverage of the roll upon which the dampening fluid is sprayed. If higher flow rates of water or dampening fluid are needed, various systems are in place to ensure even distribution of water or dampening fluid on the surface of the roll or cylinder. For example, in U.S. Pat. No. 4,815,375, a complex mechanical system is described which allows the spacing between adjacent nozzles to be adjusted, so that there is no overlap in the droplet spray patterns from adjacent nozzles, which would result in uneven dampening. Other prior art arrangements use pulsing of the water or dampening fluid, together with smoothing rollers downstream of the spray nozzles, in order to ensure smooth dampening across the roll or cylinder.
The present invention is a press dampening system which applies dampening fluid to a roll or cylinder using high-density straight streams of fluid flow. As used herein, the term straight stream encompasses a stream of fluid which is primarily laminar, and which produces a minimal amount of spray or droplets. Therefore, in the present invention, the flow of fluid is not, or is minimally, broken up into droplets. In the present invention, the straight streams, which include a high density of water or dampening fluid, impact against the roll or cylinder and thereafter the straight stream spreads out across the surface of the roll or cylinder in a puddle-like fashion. Each straight stream is created by an orificexe2x80x94preferably a micro-orificexe2x80x94which, at an appropriate back pressure upstream of the orifice, causes the water or dampening fluid to emerge from the orifice in a straight stream of relatively laminar flow.
A series of orifices can be spaced across the length of a pressure manifold, so that a series of straight streams are created along the length of the roll or cylinder to be dampened. The orifices are preferably micro-orifices, and the back pressure is preferably regulated by control valves. The laminar flow out of the orifices, which create a puddle-like spread of the water or dampening fluid across the surface of the roll or cylinder, allows the fluid to spread across the entire length of the surface of the roll or cylinder. The puddle-like spread of water or dampening fluid across the surface of the roll or cylinder allows the flow of each straight stream to merge into flow of adjacent straight streams with little fluctuation in the quantity of water or dampening fluid across the roll or cylinder. As a result, the present invention enhances the print quality of the printing press, by reducing print quality fluctuations which may be caused by dampening fluid fluctuations on the surface of the roll or cylinder. Furthermore, because the water or dampening fluid from adjacent straight streams mixes on the surface of the roll or cylinder in a puddle-like fashion, the present invention provides advantages over prior art spray-dampening systems.
With prior art spray dampening systems, which rely upon spray droplets to dampen a roll or cylinder, variations in the rate of application of the water or dampening fluid causes overlaps or gaps in the conical or fan-like spray patterns from the spray orifices. In order to prevent gaps or overlaps, complicated mechanical systems are needed to adjust the spacing of the spray orifices relative to one another in prior art systems. In other prior art systems, smoothing rollers are needed to smooth the dampening fluid across the width of the roll or cylinder. In the present invention, because the water or dampening fluid spreads in a puddle-like fashion and mixes on the roll or cylinder surfacexe2x80x94as a result of the straight streamsxe2x80x94there is no need to adjust spacing between orifices or to smooth the dampening fluid across the roll or cylinder.